Recently, there has been an increasing interest in energy storage technology. As the application fields of energy storage technologies have been extended to cellular phones, camcorders, notebook computers, PCs and electric cars, efforts have been increasingly been made towards the research and development of electrochemical devices. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, extensive research and development of such batteries are focused on the designs of new electrodes and batteries to improve capacity density and specific energy.
Many secondary batteries are currently available. Among these, lithium secondary batteries developed in the early 1990's have drawn particular attention due to their advantages of higher operating voltages and much higher energy densities than conventional aqueous electrolyte-based batteries, for example, Ni-MH, Ni—Cd, and H2SO4—Pb batteries.
Generally, lithium secondary batteries consist of an anode comprising an anode active material layer, a cathode comprising a cathode active material, a separator interposed between the anode and the cathode to electrically insulate them, and a non-aqueous electrolyte solution comprising an electrolyte salt and an organic solvent.
Among these, the separator should generally satisfy the requirements of safety and heat resistance to the components of batteries, high electrolytic conductivity, and sufficient strength which can maintain its original form during its preparation, processing and application in batteries for preventing contact between both electrodes. As such a separator, a polyolefin-based porous substrate having multiple fine pores has been conventionally used.
Meanwhile, the separator may have a porous coating layer comprising inorganic particles for ensuring safety, and also may further have a functional coating layer for improving adhesion of the separator with electrodes or preventing early meltdown and shutdown of the separator during overcharging. Conventional separators applying such a functional coating layer have been prepared by coating a polymer slurry in a solvent on a porous substrate or a porous coating layer, followed by drying. In the slurry used in coating, the solvent is for obtaining flexibility as well as proper dispersion and viscosity of polymer particles. However, the use of such a solvent incurs costs which may also increase depending on the needs of the solvent, and in the case the solvent is harmful to the human body, there may be additional costs in handling and storage and since the solvent requires drying after coating, a production yield may decrease.